If your ad is performing great, you can extend the ad past the original end date.
You aren't able to edit targets once they've launched, but you can add new campaigns to the existing ad.
To do this, navigate to your campaign dashboard (the "edit" link under the ad's title), and click the orange "+ new campaign" button.
If you want to extend a running ad, create a new campaign on the ad's edit page. Select a start date that is immediately after the original campaign ends. The ad will run without being interrupted. You can also make the timing overlap if you want to increase traffic to the running ad.